1. Field of the Invention
The invention relates to a vehicle steering system that is used in a material handling vehicle such as a forklift.
2. Discussion of Background
Rear wheels of a forklift are used as steered wheels. When the forklift turns a corner, a steering operation specific to the forklift is performed, that is, first, the front (fork portion) of the forklift is slightly turned in a turning direction by operating a steering member by a small steering angle and then the rear of the forklift is caused to swing out toward the outer side of the corner through a quick steering operation.
A hydraulic power steering system or an electric power steering system is used as a steering system of a forklift. With the hydraulic power steering system, in response to an operator's steering operation, a hydraulic pump is driven by an electric motor to supply hydraulic fluid to a hydraulic cylinder and a piston of the hydraulic cylinder is thereby moved to turn the steered wheels. With the electric power steering system, in response to an operator's steering operation, a rack shaft for the rear wheels and tie rods coupled to the rack shaft are moved by driving an electric motor to turn the steered wheels.
The forklift employs a so-called steer-by-wire power steering system in which a steering member and the rear wheels are not mechanically connected to each other. In this case, in order to give the operator a feel of reaction force, reaction force that should be returned from the rear wheels to the steering member is created on the basis of a steering angle. Japanese Patent Application Publication No. 2006-298275 (JP 2006-298275 A) describes a steering control device that controls steering reaction force.
FIG. 9 shows the conventionally employed correlation between a reaction force of a forklift and a steering angle θh. The abscissa axis represents the steering angle θh, and the ordinate axis represents the reaction force. Under this correlation, the flow of a steering operation at the time of making a right turn at a right-angle corner is as follows.
(1) At an initial stage (around 0 degrees) of the turn, the front (fork portion) is directed slightly to the right by operating the steering member by a small steering angle. At this time, the steering angle θh is within a range indicated by “A” in FIG. 9, and the reaction force received by the operator is small.
(2) In the first-half of the turn, the rear of the forklift is caused to swing out toward the outer side of the corner through a quick steering operation. At this time, the steering angle θh is within a range indicated by “B” in FIG. 9, and the operator receives a large reaction force. The reaction force is given to the operator in order to prevent an unintentional increase in the steering angle due to an excessive force in steering operation. The operator rotates the steering member against the reaction force.
(3) In the second-half of the turn, the steering direction should be reversed to cause the rear of the forklift to be directed straight ahead through a rapid steering operation. If the return of the steering member is delayed, it is difficult to cause the forklift to be directed straight ahead. For example, the forklift excessively turns beyond a right angle, which may lead to, for example, an accidental contact.
The steering angle range of the forklift is wide and a steering operation is frequently performed in the forklift, due to the nature of its use. Therefore, the operator needs to operate the steering member by a large amount, so a large burden is placed on the operator. Therefore, there is a need for control for smoothly returning the steering member when the operator reverses the steering direction through a rapid steering operation in the second-half of the turn in the above description (3).